1. Field of the Invention
The invention relates to an exhaust gas control apparatus and method for an internal combustion engine, which purifies exhaust gas using a NOX storage-reduction catalyst.
2. Description of the Related Art
An exhaust gas control apparatus has been proposed which purifies NOX in exhaust gas using a NOX storage-reduction catalyst provided in an exhaust passage. For example, Japanese Patent Application Publication No. JP-A-2004-84617 describes technology that executes rich spike control (hereinafter also referred to as “RS control”) which makes the air-fuel ratio rich in order to reduce NOX stored in the NOX storage-reduction catalyst.
In addition, technology has also been proposed to purify NOX in the exhaust gas using ammonia (NH3). Japanese Patent Application Publication No. JP-A-10-47041 describes technology in which a NOX storage-reduction catalyst is arranged in some of the exhaust ports. NH3 is produced by making the air-fuel ratio of the exhaust gas in the cylinders upstream of the NOX storage-reduction catalyst rich and is used to purify NOX discharged from the other cylinders in the downstream exhaust gas control catalyst. Also, Japanese Patent Application Publication No. JP-A-2004-218475 describes technology which improves NOX purifying performance in both low and high temperature regions by providing a NOX selective reduction catalyst upstream of a NOX storage-reduction catalyst and adding urea into the exhaust passage. In addition, Japanese Patent Application Publication No. JP-A-2005-214098 describes technology that produces NH3 by making the air-fuel ratio of the exhaust gas at the NOX catalyst portion rich and purifies NOX using that NH3 during lean burn operation.
However, with the technology described in the Japanese Patent Application Publication No. JP-A-2004-84617, there were cases in which NOX components leaked out from the NOX storage-reduction catalyst (hereinafter this may also be referred to as simply “NOX leakage”) in the initial stage of RS control. Such NOX leakage is thought to occur when reducing agent components such as HC, CO, and H2 are consumed by oxygen or the like such that exhaust gas with an insufficient amount of the reducing agent is supplied to the NOX storage-reduction catalyst. Also, with the technologies described in the Japanese Patent Application Publications No. JP-A-10-47041, JP-A-2004-218475, and JP-A-2005-214098, as well, it was difficult to appropriately suppress such NOX leakage from the NOX storage-reduction catalyst when reducing NOX therein.